close vs shutdown socket

Question

In C  I understood that if we close a socket  it means the socket will be destroyed and can be re-used later   How about shutdown  The description said it closes half of a duplex connection to that socket  But will that socket be destroyed like close system call

User · Accepted Answer

This is explained in Beej s networking guide   shutdown is a flexible way to block communication in one or both directions   When the second parameter is SHUT RDWR  it will block both sending and receiving  like close    However  close is the way to actually destroy a socket   With shutdown  you will still be able to receive pending data the peer already sent  thanks to Joey Adams for noting this

User · Answer

Close  When you have finished using a socket  you can simply close its file descriptor with close  If there is still data waiting to be transmitted over the connection  normally close tries to complete this transmission  You can control this behavior using the SO LINGER socket option to specify a timeout period  see Socket Options    ShutDown  You can also shut down only reception or transmission on a connection by calling shutdown   The shutdown function shuts down the connection of socket  Its argument how specifies what action to perform   0 Stop receiving data for this socket  If further data arrives  reject it   1 Stop trying to transmit data from this socket  Discard any data waiting to be sent  Stop looking for acknowledgement of data already sent  don   t retransmit it if it is lost   2 Stop both reception and transmission    The return value is 0 on success and -1 on failure

User · Answer

shutdown   doesn t actually close the file descriptor   it just changes its usability  To free a socket descriptor  you need to use close    1

User · Answer

None of the existing answers tell people how shutdown and close works at the TCP protocol level  so it is worth to add this   A standard TCP connection gets terminated by 4-way finalization    Once a participant has no more data to send  it sends a FIN packet to the other The other party returns an ACK for the FIN  When the other party also finished data transfer  it sends another FIN packet The initial participant returns an ACK and finalizes transfer    However  there is another  emergent  way to close a TCP connection    A participant sends an RST packet and abandons the connection The other side receives an RST and then abandon the connection as well   In my test with Wireshark  with default socket options  shutdown sends a FIN packet to the other end but it is all it does  Until the other party send you the FIN packet you are still able to receive data  Once this happened  your Receive will get an 0 size result  So if you are the first one to shut down  send   you should close the socket once you finished receiving data   On the other hand  if you call close whilst the connection is still active  the other side is still active and you may have unsent data in the system buffer as well   an RST packet will be sent to the other side  This is good for errors  For example  if you think the other party provided wrong data or it refused to provide data  DOS attack    you can close the socket straight away   My opinion of rules would be    Consider shutdown before close when possible If you finished receiving  0 size data received  before you decided to shutdown  close the connection after the last send  if any  finished  If you want to close the connection normally  shutdown the connection  with SHUT WR  and if you don t care about receiving data after this point  with SHUT RD as well   and wait until you receive a 0 size data  and then close the socket  In any case  if any other error occurred  timeout for example   simply close the socket    Ideal implementations for SHUT RD and SHUT WR  The following haven t been tested  trust at your own risk  However  I believe this is a reasonable and practical way of doing things   If the TCP stack receives a shutdown with SHUT RD only  it shall mark this connection as no more data expected  Any pending and subsequent read requests  regardless whichever thread they are in  will then returned with zero sized result  However  the connection is still active and usable -- you can still receive OOB data  for example  Also  the OS will drop any data it receives for this connection  But that is all  no packages will be sent to the other side   If the TCP stack receives a shutdown with SHUT WR only  it shall mark this connection as no more data can be sent  All pending write requests will be finished  but subsequent write requests will fail  Furthermore  a FIN packet will be sent to another side to inform them we don t have more data to send

User · Answer

linux  shutdown   causes listener thread select   to awake and produce error  shutdown    close    will lead to endless wait  winsock  vice versa - shutdown   has no effect  while close   is successfully catched

User · Answer

in my test   close will send fin packet and destroy fd immediately when socket is not shared with other processes  shutdown SHUT RD  process can still recv data from the socket  but recv will return 0 if TCP buffer is empty After peer send more data  recv will return data again   shutdown SHUT WR will send fin packet to indicate the Further sends are disallowed  the peer can recv data but it will recv 0 if its TCP buffer is empty  shutdown SHUT RDWR  equal to use both SHUT RD and SHUT WR  will send rst packet if peer send more data

User · Answer

There are some limitations with close   that can be avoided if one uses shutdown   instead   close   will terminate both directions on a TCP connection  Sometimes you want to tell the other endpoint that you are finished with sending data  but still want to receive data   close   decrements the descriptors reference count  maintained in file table entry and counts number of descriptors currently open that are referring to a file socket  and does not close the socket file if the descriptor is not 0  This means that if you are forking  the cleanup happens only after reference count drops to 0  With shutdown   one can initiate normal TCP close sequence ignoring the reference count   Parameters are as follows   int shutdown int s  int how      s is socket descriptor   int how can be   SHUT RD or 0     Further receives are disallowed  SHUT WR or 1     Further sends are disallowed  SHUT RDWR or 2     Further sends and receives are disallowed

User · Answer

This may be platform specific  I somehow doubt it  but anyway  the best explanation I ve seen is here on this msdn page where they explain about shutdown  linger options  socket closure and general connection termination sequences   In summary  use shutdown to send a shutdown sequence at the TCP level and use close to free up the resources used by the socket data structures in your process  If you haven t issued an explicit shutdown sequence by the time you call close then one is initiated for you

User · Answer

I ve also had success under linux using shutdown   from one pthread to force another pthread currently blocked in connect   to abort early    Under other OSes  OSX at least   I found calling close   was enough to get connect   fail

[c] close vs shutdown socket?

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